1. Field of the Invention
The present invention relates to a developing agent for an electronograph used for formation of an image by visualizing a static latent image of an electronographic mode or the like, and a method of manufacturing such a developing agent.
2. Description of the Related Art
In general, a developing agent is made by mixing magnetic powder, coloring agent, etc., into melted resin, and pulverizing the mixture resin by a pulverizing method including a clarification step. With such a conventional method, it is difficult to disperse the magnetic powder and the coloring agent uniformly into the resin, and the non-uniform dispersion causes the deterioration of an image. In place of the conventional pulverizing method, various types of methods of manufacturing a developing agent, such as the suspension polymerization method, emulsion polymerization method, and polymerization methods have been recently proposed. In the polymerization method, for example, a monomer, coloring agent, surfactant, and other additives are dispersed in an aqueous solution, and the mixture solution is stirred at a predetermined temperature, concentration and driving force, in order for a polymerization reaction, thereby obtaining toner. With this method, dispersion of the coloring agent particles can be performed to a certain degree.
However, thus obtained polymerization toner entails a drawback of insufficient cleaning since the particles of the toner has a sphere shape. In order to solve the drawback, there has been an attempt to make the particles of the toner non-uniformly shaped. The term "non-uniformly shaped" used here is meant to be not having a specific shape such as sphere, and the non-uniformly-shaped toner is meant to be a type containing particles of various shapes.
Such an non-uniformly-shaped toner can be prepared in the following steps as is disclosed in Published Unexamined Japanese Patent Application No. 2-187768.
(1) Formation of primary particles by polymerization
A monomer, a pre-polymerized resin emulsion, a coloring agent, an additive, and the like are dispersed or suspended in a solvent, and the mixture is stirred at a predetermined temperature, concentration and driving force for a polymerization reaction, thereby forming primary particles having particle diameters of 10 .mu.m or less. FIG. 1 shows models of particles. As can be seen in FIG. 1, the primary particles have substantially a spherical shape.
(2) Formation of secondary particles by granulation
The primary particles are granulated by a general method so as to form secondary particles having particle diameters of 25 .mu.m or less. As can be seen in FIG. 1, the secondary particles are aggregates having non-uniform shapes.
(3) Formation of associated particles by ripening
The primary particles constituting each secondary particle are fused with each other by heat, thereby obtaining non-uniformly-shaped associated particles as shown in FIG. 1.
The non-uniformly-shaped associated particles are subjected to a washing/removal-of-water/drying step, and a surface-treatment step, and then to a classification step, thus forming a non-uniformly-shaped toner having predetermined particle diameters.
In order to improve the cleaning property of this toner, the state of primary particles fused with each other may weakened by heat in the ripening step. With such a technique, there can be obtained a further non-uniformly-shaped toner having a high shape coefficient. However, if the fusion state of the primary particles is weak, the anti-crush property of the toner is degraded, causing drawbacks such as creation of fine particles and scattering of toner.
On the other hand, in order to improve the anti-crush property, the material may be heatear sufficiently in the ripening step so as to strengthen the fusion of primary particles with each other. However, as the material is heated, the particles of the toner become closer to perfect spheres, thus lowering the shape coefficient. As a result, its cleaning property decreases. With the conventional method, it is difficult to achieve non-uniformly-shaped toner excellent in both cleaning and anti-crush properties.
In each of the above steps of formation of the primary particles, granulation of the secondary particles, and formation of the associated particles by ripening, it is necessary to control the diameters of particles, and the distribution of the diameters, resulting in complication of the polymerization toner manufacture method. The dispersion property of the coloring agent in the polymerization toner is an important factor for enhancing the polymerization stability, the coloring ability, the transparency, which is necessary especially in color toner. For the dispersion step in polymerization of toner, there have been proposed various types of techniques for the purpose of enhancing the dispersion property of the coloring agent with respect to the monomer.